This invention relates to apparatus and methods for nondestructive testing, and is concerned in particular with the use of ultrasonic techniques in nondestructive testing.
Nondestructive methods are preferable for test situations where the comprehensive evaluation of inservice machinery, assembled components, or new products is required. Nondestructive techniques are particularly desirable, for example, in testing the materials employed in structures such as tracked vehicle rails, vessels and conduits for pressurized gas or steam, and pipelines. In such applications, it is imperative that the test method utilized reveal any significant flaws or imperfections so that potentially costly equipment failures and other undesirable consequences may be prevented.
Among the various techniques of nondestructive testing which are available, noncontact test techniques, which do not require physical contact between the testing apparatus and the item tested, are especially advantageous, since such techniques may be implemented with the additional advantages of high speed operation, good performance in extreme temperature environments, the ability to operate in inaccessible locations by remote control, and a need for only minimal subsequent cleanup operations.
One particular testing application, for example, in which nondestructive, noncontact testing techniques are useful is the inspection of a pipeline built from sections of pipe which are welded together and which are manufactured with longitudinal welded seams. A somewhat detailed discussion of the pipeline testing situation will serve to illustrate the state of the art, and its limitations, for nondestructive testing in general.
An inspection of the structural integrity of such a pipeline has commonly been performed at the pipe manufacturing location for the longitudinal weld and at the pipeline installation site for the girth welds joining the sections of pipe. A detailed x-ray examination of the girth welds is often performed before a pipeline is buried, but inspection of the longitudinal welds is typically deferred until a hydraulic pressure test is performed on a sample section of the installed pipeline. A failure in the pressure test can frequently be traced to cracks or other defects in or near the weld which escaped detection in earlier inspections or which were formed in the pipe during delivery, storage, or handling of the pipe prior to installation in the pipeline.
Since the hydraulic test is relatively expensive and further is not performed until after the completion of the pipeline construction phase, the cost of an earlier nondestructive inspection could be justified where such a test would ensure the detection of any defects in or near the longitudinal pipe weld which were large enough to cause a failure of the hydraulic pressure test.
Noncontact ultrasonic testing procedures and apparatus are known which potentially could be used in such a testing environment. Electromagnetic acoustic transducers (EMATs), for example, some of which are disclosed in U.S. Pat. Nos. 3,850,028 and 4,127,035, may be utilized to generate an ultrasonic wave in an electrically conductive or magnetic material through the interaction of a static magnetic field and a dynamic electromagnetic field. Cracks or other defects which are present in the material will affect the transmission or reflection characteristics of the waves in the material. Those changes may be measured, by an EMAT or other suitable transducer, and utilized in characterizing the part as acceptable or unacceptable for its intended use. Ultrasonic testing procedures have been adapted for use in pipeline inspections, as disclosed, for example, in U.S. Pat. No. 4,092,868.
Difficulties have been encountered, however, when such ultrasonic test methods are employed in an attempt to evaluate an object which contains a known discontinuity or inhomogenity. Such a discontinuity might be due to a weld, as in the type of pipeline discussed above, or might be caused by some other similar aspect of the structure of an object. It has been found that such a discontinuity tends to cause a disturbance in the reflection and refraction of ultrasonic waves which is so large that the effects due to the presence of cracks or defects near the discontinuity tend to be obscured, effectively rendering such defects undetectable by prior art ultrasonic methods.
Consequently, a need has developed in the art for an ultrasonic nondestructive testing technique which is capable of detecting flaws or imperfections in or near a known discontinuity or inhomogeneity in a material.